Persistent high latitude amplification of the Pacific Ocean over the past 10 million years

While high latitude amplification is seen in modern observations, paleoclimate records, and climate modeling, better constraints on the magnitude and pattern of amplification would provide insights into the mechanisms that drive it, which remain actively debated. Here we present multi-proxy multi-site paleotemperature records over the last 10 million years from the Western Pacific Warm Pool (WPWP) – the warmest endmember of the global ocean that is uniquely important in the global radiative feedback change. These sea surface temperature records, based on lipid biomarkers and seawater Mg/Ca-adjusted foraminiferal Mg/Ca, unequivocally show warmer WPWP in the past, and a secular cooling over the last 10 million years. Compiling these data with existing records reveals a persistent, nearly stationary, extratropical response pattern in the Pacific in which high latitude (~50°N) temperatures increase by ~2.4° for each degree of WPWP warming. This relative warming pattern is also evident in model outputs of millennium-long climate simulations with quadrupling atmospheric CO2, therefore providing a strong constraint on the future equilibrium response of the Earth System.

the age uncertainties of some sites (Sites 594, 1125, 1208, 883/884, 887 and 846) have been reported by Herbert et al. 6 , these uncertainties were obtained from assigned age uncertainties for the orbital stratigraphy, polarity sequence and biostratigraphy, which were treated as different constants at varying time intervals. Also, the age uncertainties of the other study sites are difficult to determine. Therefore, instead of using York Regression which requires the uncertainty of both x and y to be known, we applied the OLS regression to estimate linear SST trend from the proxy data. The linear SST trend over the past 10 Myr (Supplementary Fig. 10) shows amplified warming in the high latitude relative to the WPWP. The p-value were calculated in the linear regression to evaluate the significance of warming trend. The paleo-results suggest that except Sites 882 (pvalue = 0.40) and 887 (p-value = 0.11) which were limited by the number of data, all other sites show significant warming in the Mio-Pliocene ( Supplementary Fig. 10).

Supplementary note 3. Single-site relative to high latitude SST change
We applied York Regression to obtain specific-site SST relative to high latitude SST change between 9.8 and 0.5 Ma ( Supplementary Fig. 13, 14 and 15). This is performed by using high-latitude SST as x-axis and binned SST of individual site as y-axis. In addition to the WPWP and middle latitude sites discussed earlier, Site 1146 was also included in this analysis because of the available Mg/Ca-SST data covering 10-5 Ma 2,7 , although it does not reside in any of our climate zonation classifications (WPWP, middle latitude or high latitude). The reported data were adjusted to the modeled SH98 Mg/Casw scenario 8 to be consistent with our Site 806 SSTs (Fig. 2). Similarly, we applied the OLS regression to the model outputs from mid-Pliocene simulations and abrupt quadrupled-CO2 (4×CO2) simulations and yielded specific-site SST change relative to high-latitude SST change over the whole time series of the model simulation ( Supplementary Fig. 13,   14 and 15).

Supplementary note 4. Data vs. models
In terms of the equilibrium mid-Pliocene simulations, the comparison between CESM1.2 and CESM2 reveals that the Pacific warming pattern derived from CESM2 agree better with the 10-Myr paleoclimate record than that from CESM1.2 ( Fig. 5 and Supplementary Fig. 13), although CESM2 generates higher SST in the WPWP than the reconstructed one (Fig. 5). The Pacific high latitude amplification factor derived from the mid-Pliocene CESM1.2 simulation (2.68 ± 0.30) is nearly identical to that from the Neogene record (2.42 ± 0.64), substantially higher than the value (1.61 ± 0.11) from the CESM2 simulation ( Fig. 5 and Table 1). For the site-specific relative to high latitude SST change, CESM1.2 simulation generated smaller values than CESM2 which yielded comparable values with those from the 10-Myr record in the WPWP and middle latitudes ( Supplementary Fig. 13).
Here we also evaluate if millennial-length climate simulations with abrupt 4×CO2 forcing 9 are capable to reproduce the middle and high latitude amplification pattern observed in the Pacific for the past 10 Myr. Besides CESM104 (Fig. 5), CNRMCM61, GISSER2, HadCM3L, and MPIESM12 also reproduce the observed middle and high latitude amplification well ( Fig. 6 and Supplementary Table 3). Although the Pacific high latitude amplification factors derived from CCSM3, HadGEM2, IPSLCM5A, and MPIESM11 slightly deviate from the Neogene value, the temperature variations from these models are within the Neogene range. In contrast, the WPWP, mid-and high-latitude temperature changes from FAMOUS are larger than those observed for the last 10 Myr, which is expected due to its high climate sensitivity (8.55°C/doubling, Supplementary Table 3), despite that the magnitude of high latitude amplification derived from FAMOUS agrees with the Neogene data. ECHAM5 also produces larger SST change in the WPWP than the 10-Myr record while the mid-and high latitude temperature changes from ECHAM5 are smaller compared with the 10-Myr record.
For the single-site relative to high latitude SST change ( Supplementary Fig. 13, 14 and 15), all the models generate similar results to the Neogene data in the WPWP except ECHAM5 with higher values and CCSM3 with lower values. For the sites in the EEP, no model is capable of fully reproducing the Neogene records. For the mid-latitude sites, CCSM3 and HADCM3L produce lower SST changes than the Neogene data; CESM104 and ECHAM5 yield similar results to the Neogene data at four sites, and the other models do not completely reproduce the reconstructed records at all four sites ( Supplementary Fig. 15). Overall, all the models except CCSM3, ECHAM5 and FAMOUS well reproduce the pattern of the middle and high latitude amplification found in the Pacific for the past 10 Myr.

Supplementary note 5. Pacific high latitude amplification vs. Arctic amplification
Arctic amplification (AA) factor is commonly defined by the ratio of the Arctic (67.5°N to 90°N) temperature change to the global average temperature change 10 . Here we evaluate the relationship between AA factor and Pacific high latitude amplification factor defined in our study, based on CESM/CAM5 and CESM/CAM6 simulations, and millennial-length simulations with abrupt 4×CO2 forcing from different climate models 9 . We calculated CSEM/CAM5-derived surface warming in the year 2000 with the present-day CO2 forcing or 800 ppm CO2 forcing relative to the pre-industrial (PI), mid-Miocene relative to PI, and CSEM2/CAM6-derived surface warming from present day (1×CO2) to instantaneously doubled CO2 (2×CO2) forcing ( Supplementary Fig. 16). The comparison between different amplification factors shows that AA factor is moderately larger than the Pacific high-latitude amplification factor, as expected ( Supplementary Fig. 16). In addition, surface air temperature (tas) anomaly over the last 30 years of each simulation with abrupt 4×CO2 forcing (Supplementary Table 4) reveal that Pacific high latitude amplification is weaker than AA in nine models except CCSM3 and FAMOUS, which retain larger high latitude amplification factor than other models. Based on these comparisons, the AA factor for the past 10 Myr, although it cannot be constrained due to lack of SST records from the Arctic Ocean, is likely to be slightly higher than 2.42 ± 0.64, the Pacific high latitude amplification factor obtained from the paleoclimate data. Similarly, simulations summarized in the Fifth Assessment Report of the IPCC have predicted that the Arctic will warm 2.